1. Field of the Invention
The present invention relates to a mirror composed of multi-layered film for use in a soft X-ray region, and more particularly a multi-layered mirror adapted for use in a soft X-ray microscope for organism observation.
2. Related Background Art
The refractive index of a substance in the X-ray region is represented by n=1-.delta.-ik wherein .delta. and k are real numbers much smaller than unity (the imaginary part ik represents the X-ray absorption). For this reason, lenses based on refractive action, as in the visible wavelength region, cannot be utilized in the X-ray region.
Consequently there is utilized a reflective optical system. However, since the reflectivity is very low at an incident angle smaller than the totally reflecting limit angle .theta..sub.c (about 5.degree. or smaller for a wavelength of 25 .ANG.), there is employed a multi-layered mirror having a plurality of (for example several hundred) reflecting planes formed by laminating layers of substances of a combination showing a high amplitude reflectivity at the interface and regulating the thicknesses of said layers in such a manner that the reflected waves mutually match in phase according to the optical interference theory.
More specifically, such multi-layered mirror can be obtained by alternately laminating a substance showing a larger difference in refractive index from vacuum (refractive index=1) and another substance showing a smaller difference. There are conventionally known certain examples of combinations of such substances, such as W (tungsten)/C (carbon) and Mo (molybdenum)/Si (silicon), and such mirrors have been prepared by thin film forming technologies such as sputtering, vacuum evaporation, CVD etc.
The wavelength of X-ray employed for observation of living organism in a soft X-ray microscope is selected in a region called "water window" wherein proteins and water show a large difference in absorption coefficients as shown in FIG. 6, namely a region between the K absorption edge (23 .ANG.) of oxygen and the K absorption edge (44 .ANG.) of carbon. A wavelength of 25 .ANG., close to the absorption edge of oxygen showing a smaller absorption coefficient, is preferred in order to enable observation of a thicker specimen.
Also the periodic thickness d (combined thickness of a pair of said substance having a larger difference in refractive index from vacuum and said substance having a smaller difference), shown in FIG. 1, has to approximately satisfy the Bragg's law 2d.multidot.sin .theta.=.lambda., wherein .lambda. and .theta. are respectively the wavelength of X-ray and the angle between the mirror surface and the incident X-ray. The conventional multi-layered mirror has been designed to reflect the X-ray in said "water window" region by suitably selecting said periodic thickness and said angle of the incident X-ray to the mirror surface.
In a soft X-ray microscope or the like, the size of the multi-layered mirror can be made smaller and the freedom in the optical system design becomes larger, if the X-ray is made to enter the mirror as perpendicularly as possible. With such mode of entry, the microscope itself can be made more compact. Stated differently, said angle of the incident X-ray to the mirror surface should preferably be as large as possible.
However, as will be apparent from the foregoing Bragg's law, the periodic thickness has to be made smaller in order to introduce the X-ray, within the above-mentioned wavelength region, into the multi-layered mirror in a state as close to the perpendicular entry as possible. For example, the periodic thickness has to be 24 .ANG. in order to introduce the X-ray of a wavelength of 33.7 .ANG. with an angle of 45.degree. to the multi-layered mirror. Thus, there has been a need for a multi-layered mirror having a high reflectivity and a reduced periodic thickness.
Nickel has a large difference from vacuum in refractive index in the above-mentioned wavelength region, and is therefore expected to provide a high reflectivity. In practice, however, in a multi-layered film employing nickel, it has been experimentally confirmed that the reflectivity rapidly drops when the thickness of nickel layers (or periodic thickness of the mirror) is reduced.
FIG. 3 shows the relationship between the periodic thickness and the ratio of measured reflectivity to calculated reflectivity when X-ray of a wavelength of 1.54 .ANG. is reflected by a multi-layered film consisting of nickel/silicon oxide. Said multi-layered film provides a reflectivity corresponding to 80% of the calculated value at a periodical thickness of 60 .ANG., but rapidly loses the reflectivity with the reduction in the periodical thickness, and becomes totally incapable of reflection when the periodical thickness reaches 30 .ANG.. Thus even the multi-layered film employing nickel loses reflectivity with the reduction in the periodical thickness, and is incapable of providing a high reflectivity in a state of a large angle of the incident X-ray to the mirror surface, in the aforementioned "water window" wavelength region.